President Bush called yesterday for a constitutional amendment restricting marriage to the union of men and women, asserting that gay marriage would weaken society.

Bush's support for a ban on gay marriage put the weight of the White House behind Christian conservatives working to thwart a Massachusetts court ruling that could permit same-sex wedding vows as soon as May 17, and the subsequent decision by San Francisco to issue marriage licenses to more than 3,200 gay couples so far.

"After more than two centuries of American jurisprudence, and millennia of human experience, a few judges and local authorities are presuming to change the most fundamental institution of civilization," he said. "Their actions have created confusion on an issue that requires clarity."

To become part of the Constitution, an amendment must win a two-thirds vote in each chamber of Congress and be ratified by 38 states, a process likely to take years. Opponents pointed out that this would be the only time a constitutional amendment had restricted rather than expanded liberties besides the since-repealed prohibition on the sale of intoxicating liquor in 1919.

Bush said he wants to preserve marriage as a union of one man and one woman but allow state legislatures to determine whether same-sex couples should receive various benefits, a formula that apparently would allow the kind of civil unions and domestic partnership arrangements that exist in Vermont and California.

Such arrangements confer some rights of marriage but are not recognized by the federal government or other states. That means gay partners would not receive Social Security survivors' benefits, inherit property without paying federal taxes, or be guaranteed the same pension benefits or hospital visitation rights as married couples.

"Marriage cannot be severed from its cultural, religious and natural roots without weakening the good influence of society," Bush said. "Government, by recognizing and protecting marriage, serves the interests of all."

Polls show the nation is split on the question. A Washington Post-ABC News poll released yesterday found that 46 percent of respondents favored an amendment banning gay marriage and 45 percent opposed it.

But with the issue of gay marriage heavily in the news, support for such an amendment rose 8 percentage points from a similar poll taken last month.

Republican leaders on Capitol Hill had discouraged the White House from embracing the issue and expressed severe reservations about the feasibility of passing an amendment.

Bush's aides did not say how much muscle he would put into the fight. Tony Perkins, president of the Family Research Council, a leading voice of social conservatives, said his group expects Bush to lobby as hard for the amendment as he did for tax cuts and a prescription drug benefit for Medicare. "I don't think that they would have just thrown this out there as a political gambit in an election year," Perkins said.

Bush, who said he wanted to "conduct this difficult debate in a manner worthy of our country, without bitterness or anger," was not specific about what he wanted to see in an amendment. White House press secretary Scott McClellan told reporters that a version introduced by Rep. Marilyn Musgrave (R-Colo.), which is called the Federal Marriage Amendment and is backed by key evangelical conservative groups, "meets his principles." Signaling the desire for changes, McClellan said the administration will "work closely with Congress on the specifics."

Musgrave's proposed amendment says: "Marriage in the United States shall consist only of the union of a man and a woman. Neither this constitution or the constitution of any state, nor state or federal law, shall be construed to require that marital status or the legal incidents thereof be conferred upon unmarried couples or groups."

Musgrave and the Alliance for Marriage, a group of religious leaders who have been pushing that language since 2001, say the amendment would ban gay marriage but still let each state decide whether to allow civil unions. But a number of constitutional scholars, and even a few of the amendment's authors, say it would effectively block some or all civil unions.

The debate over gay marriage puts a social controversy at the forefront of the nation's politics. Democrats said constant attention to the issue could hurt them in statewide races by elevating cultural issues over economic ones in the South and in the swing states of the Midwest.

Some Republicans expressed trepidation about Bush's becoming aligned with a controversy that could make it easier for the party to be portrayed as intolerant. Officials who are close to the White House, and who declined to be named to avoid angering Bush and his inner circle, said his aides have been conflicted about joining the debate. Some aides pointed to the disagreement among advocacy groups about the effect of the Musgrave amendment and said they could not urge Bush to support something when they could not be sure what it would do.

"If the White House thought this was a clear win, they would have been out earlier, louder," one of the officials said.

But political analysts said one of the attractions of the issue for the White House is that it could help win over some traditional Democratic constituencies, including African American churchgoers and Hispanic Catholics.

The position is a shift from Bush's campaign, when he said the issue should be left to the states. "Don't try to trap me in this states' issue like you're trying to get me into," he said when asked about gay marriage during a primary debate in 2000.

Vice President Cheney, who has a gay daughter, said during the vice presidential debate that he did not "think there should necessarily be a federal policy in this area."

Bush has been talking about the issue for six months without formally endorsing an amendment, leaving some conservatives with the impression he made yesterday's announcement reluctantly. He finally announced his support for an amendment the day after launching his general election campaign by attacking his likely opponent, Sen. John F. Kerry (D-Mass.), as a waffler. Kerry was one of 14 senators to vote against the Defense of Marriage Act, which denies federal recognition of same-sex marriages and was signed by President Bill Clinton in 1996. Bush yesterday cited the possibility that the act could "be struck down by activist courts."

Campaigning in Ohio, Kerry said he saw no reason for a federal amendment banning gay marriage. He left the door open for state action on the issue, but only if states include provisions allowing for civil unions and domestic partnership rights. "I believe, as a matter of belief, that marriage is between a man and a woman," he said.

His wife, Teresa Heinz Kerry, was quoted by the Associated Press as saying that she believes the country would move toward acceptance of gay marriage. "I think with time and without a lot of politicization of this, we'll get there," she said.

Sen. John Edwards (N.C.), who trails Kerry in the race for the Democratic nomination, said in Atlanta that he opposes gay marriage and is also against the amendment. "The federal government should not be involved," he said.

Gay rights groups reacted bitterly to Bush's speech. Cheryl Jacques, president of the Human Rights Campaign, one of the country's largest gay rights groups, said she was "deeply disappointed that this president would do something as shameful as try to use our U.S. Constitution to score cheap political points to jump-start a faltering presidential campaign."

Some conservatives contended that Bush did not go far enough. Concerned Women for America issued a statement saying the group "cannot support the defective remedy he has chosen," because it could allow civil unions.